however, if you want to deepen the topic, you can read these lines extracted from a pdf which is also found in the network....and I have to say, as always, that what is written is not, in any way, claim to explain everything as c?? to know about. She wants only to be an introduction for those who are passionate about cars, technology and science, or just curious, want to explore further or learn something new.
In order to obtain a good combustion process of the fuel all?the inside of the combustion chamber, the characteristics of the charge injected are of primary importance.
The fast and ?intimate? mixing of the fuel with l?the air, as well as the ability? of this mixture take up a pi? volume can be between what's available, that are absolutely necessary to achieve that?goal.
The roads used by designers to achieve such effects are at least two:
- The care of the fluid dynamics with the study of the motions of the squish, tumble and swirl;
- The study of the formation of the spray and its characteristics.
The formation of very fine droplets ? of extreme importance. L?evaporation ? in fact, a process surface, as a result, the speed? with the same volume as the liquid passes to the state steam pu? be increased by increasing the liquid surface available. This allows for the rapid formation of the mixture of air and fuel and a pi? complete combustion.from this it is deduced that, for example, in the combustion chamber, the ratio between air and fuel can? vary from 0 within a microgoccia of unburned fuel to infinity in areas where the ec only to air that does not interact with the fuel....
The atomization process then ? very complex; it depends from?the interaction of the spray with l?air?the inside of the cylinder, which results in a balance of forces between the jet fluid and l?air. Obviously they have a major role in the physical and chemical characteristics of the liquid itself.
The study of such phenomena ? simple, and mathematical models exist for the pi? empirical.
You ? hypothesized that the formation of the spray is done in two phases:
- L?atomization primary;
- L?secondary atomization.
The surface of the liquid jet outgoing from?the nozzle is put in rapid oscillation. These oscillations, which are a function of the geometric characteristics of the nozzle, are amplified from?the interaction of the liquid with l?air. When their intensity? exceeds a certain limit value, which is the crushing of the jet, called primary atomization.
The drops cos? formed undergo l?action of different forces, which depend on the speed? of the drops themselves, (which in turn depends on pressure but also on the viscosity? the diesel), but also by other characteristics of the fuel (density, the surface tension). These forces determine a second fragmentation of the droplets, a phenomenon known as secondary atomization.
For what is written you can? to say that the formation of the spray, in addition to being a function of the geometric characteristics of the the nozzle (including the diameter of the holes, but not only) depends on:
- Injection pressure (increase the speed? of the drops but also l?intensity? of the aerodynamic forces);
- Internal pressure dell?the air in the cylinder;
- Properties? of the fuel;
But also by the length of?injection.
Of course, in addition to the formation of particles of small size, must be considered, as mentioned previously, the ability? of the spray to occupy the largest volume possible.
Of us? is taken into account through two parameters, the penetration of the spray and l?opening angle of the cone of spray.
- The penetration increases all?increase of the diameter of the droplet;
- Increases all?increase of the injection pressure (only up to a certain point since? high-pressure formation of very small particles that evaporate easily);
- Reduced back pressure (this effect ? maybe more? important-to-cone spray).
But let's see how these parameters work.
Pressure dependence:
In short, in the chart ? shown l?the trend of the length of penetration of the spray as a function of the supply pressure and injection time, for a nozzle-type VCO with holes from 200um in diameter.
As expected there ? a clear difference between the 60, 100 and 130MPa in the lengths of the spray , as well as in the angular coefficient of best-fit. Abnormal can? instead, appear l?the trend of the experimental data between 130 and 160MPa, where you do not notice any obvious differences.
There? ? probably due to the increased evaporation of the drops produced at 160MPa, which then can not penetrate long in the cylinder
Depending on the type of nozzles:
The test conducted on the two types of nozzles (VOC and mini-sac) , with the diameter of the holes is variable between 0.1 - 0.15 and 0.2 mm, showed a lower degree of penetration of the spray nozzles of the type VOC of equal size.
There? ? probably due to a bigger capacity? cold mist from the part of the VOC, which turns into a better rate of evaporation of the fuel.
L?the analysis could go on about the countless parameters. The shape of the spray ? in fact, dependent on the duration of?injection, the temperature of the?the air in the cylinder to equal? pressure (if not very), and by other factors such as the physical and chemical characteristics of the fuel.
It should be remembered that, although one can assume the conditions of density?, viscosity? and surface tension of the fuel, as the values in the standard conditions (1Atm and 25?C), the conditions change to temperatures and pressures that are typically present in an injector, including the density, for the liquid, and is commonly considered to be constant, given the typical incomprimibilit? of the liquids, but that 1600-2000 bar undergoes variations.
Not wanting to burden the discussion, which is supposed to be a valid approach for all, can you? ultimately to say that, when you make a change to the injection parameters (timing, advance injection, rail pressure), but also to pressure turbo, all you can? to say, except that to make a prediction about how it will come? changed the spray.
I hope not to have missed anything important in the attempt to simplify a speech very well articulated.
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Originally Written by megamind
